Secret signaling



Jan. 26 1926.

B. W. KENDALL SECRET SIGNALING Original Filed 1920 llAlA 220012 22oos-- I hvenfar: Bur/an W Ken/al/ by Patented J. 26, 19.

UNH. ST

BURTON W. KENDALL, OF EAST ORANGE, NEW JERSEY. A SSIGNOR TO WESTERN ELEC- TRIC COMPANY, INCORPORATED, OF NEW YORK. N. Y..-.A CORPORATION OF NEW ne a e... W...- Bani avaaaere cow SECRET-SIGNALING- Original application flled December 23, 1920, Serial No. 432,680. Divided and this application filed June a 19, 1925. Serial No. 38,129.

To all whom it may concern:

Be it known that I, BURTON W. KENDALL,

a citizen of the United States of America,

speech or intelligible sounds by electrical waves.

An object of the invention is to provide a method of and system for secret signaling.

Another object of the invention is to provide a method of transmitting audible frequency waves which are unintelligible as speech but from which intelligible speech waves may be derived.

One feature of the invention relates to modulation of carrier waves by waves of speech frequency range representing speech but unintelligible as such when detected by ordinary methods.

An additional feature of the invention is the provision of a radio telephone system which is secret to those not provided with a special receiving apparatus.

The present application is a. division of my copending application Serial No. 432,-

680, filed December 23, 1920.

Ordina complishe by transmission of electric currents which correspond in their instantanebus amplitudes and fro encies to the simultaneous amplitudes and. frequencies of the difierent sound wave components comprised in articulate speech. The essential speech frequencies may be considered to ran e from about 200 cycles to 2200 cycles. ccordingly, articulate speech is intelligible as such when it comprises the proper frequency components with their accompanying harmonics j n of tlii lfproper relative'intensities; eratlon'whi'eh dest'roys'thelsle essen'tial I V l -ships; makesfit-iin o 'si Qtd re'ceive inte' qi ere spaea tfitn in aiatma e sgaate 0 HIGH}; i530 Lit-i7 09$ smatelephone transmission is acrelationrange as wide asthat of the original speech waves, the essential relationships for intelligible speech'have been destroyed. This further involves transmission of the changed speech waves and'restoration. of the essential speech relationships at a receiving station.

Accordingto the present invention, speech waves are combinedwith a single wave of audio frequency to produce a modulated audio frequency wave'with two side bands, each having a frequency range coextensive with that of the original speechcurrents. One of thesebands includes the sum of the frequencies of the speech currentcomponents and the audible wave component, the other, thedifierence of'the' audible wave frequency and the speech component frequencies One of the two bands is selected and may be used to transm Speech enrgy though the essential speech frequency relat'ionsh'ps have 'be'en'destroyed sothat the transmitted energy, it directly appliedfto a receiver. produces "merely an unintelligible noise. Preferably, the .difierence frequency side'band is selected for transmissionfsince in thisjband the fre'que'ncies of the individual speech components have been inverted. "If the audible wave combinedwith the speech wave'has a frequencyat' approximately the, upper limit of the essential speech frequency range, the idiii'er'en'ce frequency "orflinverse frequencyband occiipy the same absoluterange of 'frequen' es asthe original speech. Increasing frequency in the "speech will be represented by decreasing frequen cv in the 'inverselfrequencyspeech band and vice versa. The unintelligible audioieband thus obtained may'betransmitted directly as inthe case of simple {electric telephony but,

infaccordance. with the invention "to be claimed a in this 5 divisional application the junintelligiblefaudioband is caused to modify forQgith d' r 'uid'd a erge H ied to deiries*ofthecharacterflof modutofs, aaeaeeeioie naeai tae aeaf r mea w t eat "we p eras hangar awn-at an cerfi ane -te e ntense a glifdio 8a (at m i itsfi'faiit titff'ii qlii an feohpa.

at aatinqmn oiaozm'xsidii on liohiw ,OQ tote-Hinze i0 Jedi oi item merely to indicate the respective elements upon which the sinusoidal electromotive forces are impressed and from which the sum and difference frequency components are derived and may, therefore, be one and the same circuit. The capital letters A, C,

F and M as eniplo ed in the drawing are used to denote amplifier, combiner, filter and modulator respectively.

In the drawing, Fig. 1 illustrates a'trans-- mitting system embodying certain features of the invention in which speech currents from an ordinary telephone line are utilized to produce an equally extensive range of inverse frequency currents which lie within the normal speech frequency range but are unintelligible; as speech when supplied to an ordinary receiver; Fig. 2-illustrates a receiving station suitable for cooperating with the transmitting station of Fig. 1; Fig. 3 shows diagrammatically the invention applied to a radio transmitting system, and .Fig. 4 shows a radio receiving station suitable for cooperating therewith; 1

Figs. 1 and 2, respectively illustrate a transmitting and a receiving station which are located at the opposite terminals of a line 3 the transmitting station having an 7 outgoing channel only and the receiving station having an incoming channel only. In

' practice, each station would usually be arranged for two-way transmission and would accordingly have both an incoming channel and an outgoing channel connected to line 3 by balanced hybrid coils in the usual manner. I

Referring to Fig. 1, the outgoing channel 6 is shown as comprising potentiometer 16, low pass filter 18, balanced modulator M, vacuum tube oscillator 20 and amplifier 21, and a second low pass filter 22 similar to filter 18. The low pass filters are of the general type disclosed in Gampbell Patent 1227,113, dated May '22, 1917, and are so designed as to transmit with practically uniformly negligible attenuation, currents of all frequencies less than a fixed cut-off frequency, as for example, 2200 cycles, while substantially extinguishing all currents of higher frequencies. The oscillator 20 comprises the well-known three-element electron discharge device having a frequency determining tuned loo consisting of inductance 24 and variable-capacity 25 in its plate-filament circuit and having its gridfilament circuit coupled by an inductance 26 to inductance 24 in Well-known manner to enable the device to produce continuous oscillations. The grid-filament circuit of thermionic amplifier 21 is connected directly tothat of oscillator 20, which aclow pass filter.

cordingly impresses the generated oscillations upon the amplifier, which in turn, impresses amplified oscillations across the variable resistance 27. The amplifier 21, oscillator 20 and balanced modulator M-are each supplied with space current from. the common source 28 through choke coils 29 which exclude the oscillatory circuit, thus prevent-' ing high frequency coupling to the common space current supply. The various filamen- .tary cathodes of the modulator, oscillator and amplifier may likewise receive heating cur- 'rent from a common source, but individual sources'have been shown to sim lify the drawing. The modulator M is o the balanced type disclosed in Hartley Patent 1,419,562 dated June 13, 1922 and, as is well known, serves to pass the modulated side bands or sum and difference frequency components of the currents impressed on 'the input circuit while substantially preventing transmission of unmodulated currents corresponding to the impressed waves. If the 0s cillator 20 supplies a wave of 2200 cycles frequency, the two side bands produced by modulation with the speech currents will both be within the audible range, and the lower side band will occupy substantially the same range as the original speech, but will be unintelligible because it consists entirely of inverse frequencies. For example, the frequencies 800, 1000 and 1200 of the original speech components will have become inverse frequencies 1400, 1200 and 1000, respectively. An increasing pitch tone of speech will become an unintelligible decreasing pitch noise in the inverse frequency band. The filter 22 suppresses the upper side band and permits transmission of only the inverse frequency band.

The receiving channel in Fig. 2 comprises substantially the same general arrangement of apparatus as in the transmitting channel of Fig. 1 with the addition of a receiving amplifier, namely, an oscillator and a second amplifier, which are connected to supply oscillations to the receiving amplifier output circuit, abalanced combining device, and a These elements correspond in tuning to those of the outgoing channel of Fig. 1, and are designated by reference characters similarto those designating the corresponding elements in that channel. The inverse frequency speech currents as received from the transmitting station are amplified bv the receiver amplifier, and maybe combined in the. balanced combining device 19' lected andtransmitted to thetelenhone circuit 2' by the low pass filter II8 whiehsuppresses the upper side band. The balanced combining device M in the outgoing channel in Fig. 1, as already explained, prevents transmission to the line 3' of ordinary speech currents. The balanced combining device C in the incoming channel of Fig. 2 similarly prevents transmission of the i11- yerse frequency currents to the telephone line 2. It follows that while both lines 2 and 3' transmit the same frequency ran e of currents, the currents of line :2 are intelligible in any ordinary receiver, while those of line 3 are not. The line 3, since it transmits speech frequencies, may be adapted for ordinary spec ch transmission as, for example, a loaded line.

In Fig. 3, the elements 18, 20, M and 22 may he the same as the corresponding elements of Fig. 1. The filter 22 feeds into a second modulator M" of any suitable type which is supplied also by a radio frequency were from an oscillator U, which may be of tr e same type as oscillator 20, but its constants are adjusted so that it produces a wave of a frequency suitable for radiation.

In l, the elements A, C, 20 and 18' same as the corresponding ele ments of ig. Goupled'between the an tenna and the amplifier A is a demodulator or combiner of any suitable The opera n of the system Figs, 3 and follows:

5 re elves a The filter 18 of desired speechinsrmssio" wave from circuit and l to some ceiyed.

Preferred vention, con hing however, limitations unessential to the invention itself, have been il lustrated and described by way example,

The scope of the invention is limited only by the appended 'claims.

What is claimed is:

1. The method of secret radio telephony which comprises producing an inverted F speech frequency band occupying approximately the same absolute frequency range as normal speech, modulating a radio wave in accordance with said inverted frequency a hand, and radiating said modulated waves,

2. The method of transmission of waves comprised of a band of frequency components which consists in interchanging the frequency positions of the various components while keeping the resultant frequencies within substantially the same limiting frequencies as the original band, and modifying a carrier wave with the resultant frequency band. y

3. The method of telephony comprising inverting the frequency order of speech waves to produce waves whose components occupy frequency positions corresponding to the difference between a constant frequency and the speech frequencies before inversion, the inverted frequency waves lying between substantially the same frequency limits as the original speech, and 1nodulat-- carrier wave by the inverted speech waves,

method --sinitting a car ier Waves of approximate q uency range, saic wave their normal freque ES i n speech occ" pyin substantia y the speech f eqi hut representing; the frequency inv normal speech, means to modulate a car i wave ey said hand of inverted frer'uen waves. means for transmitting modulated, mes s for receiving and detectine' said wave to produce speech from components therefrom, r er is for inver the speech frequency components so 1155C? duced and a telephone receiver for responding to the last mentioned inverted compo- I nents.

8. The method of transmission of waves comprised of a band of speech frequency components which consists in inverting each of the speech frequency components of said band to produce an unintelligible inverted frequency band occupying a frequency range overlapping that of the original band, modulating a carrier wave in accordance with said inverted frequency band, and radiating the modulated wave.

9. The method of transmission of waves comprised of a band of speech frequency components which consists in inverting the speech frequency components of said band so as to proudce a'resultant inverted speech frequency band occupying a frequency range overlapping that of the original speech, modulating a carrier wave with said inverted band, transmitting the modulated wave, l eceiving the modulated carrier wave, detecting said wave to produce waves of the speech'frequency range therefrom, and inverting-the last-mentioned waves.

10. In a carrier telephone system, means to invert a band of speech frequencies to produce an unintelligible inverted frequency band occupying a frequency range overlappin that of the original band, means to modulate a carrier-wave with said inverted fre uency band, and means to radiate the mo ulate wave. i

a 11. In a carrier telephone system, means for generating a carrier wave, means for producing an "inverted speech frequency band occupying a frequency range overlapping that of the original speech, means to modulate said carrier wave with said inverted speech frequency band. means for transmitting the wave so modulated, means for receiving and detecting said modulated Wave to produce speech frequency components therefrom, means for inverting the speech frequency components so produced, and a receiver for responding to the lastmentioned com onents.

In witness w ereof, I hereunto subscribe my name this 16th day of June A. D., 1925.

BURTON w. KENDALL. 

